This invention relates to lock valves which are commonly used in hydraulic control circuits to prevent the leakage of fluid from fluid motors, such as hydraulic jacks and the like, due to the influence of external pressures or forces acting on the jacks.
Typical examples of prior art lock valve devices known to Applicant are disclosed in U.S. Pat. No. 2,691,964 to Stickney; U.S. Pat. No. 3,563,261 to Griffith et al; and U.S. Pat. No. 3,857,404 to Johnson, which patent is assigned to the Assignee of this application.
Many hydraulic systems employed, for example in earthmoving vehicles and the like, use double acting hydraulic jacks for adjusting various machine components and implements carried on such vehicles. It is well known in the art, that external forces or loads acting on such jacks through such machine components may create either a negative or a positive load condition. In this regard, the phrase--negative load condition--is meant to describe a condition where such external forces are oriented in a direction to assist inlet fluid pressure to the jack from the hydraulic system, as when lowering a load in the bucket of an end loader, and the phrase--positive load condition--is used to describe a condition where such external forces are oriented to oppose the inlet fluid pressure, as when raising a load.
In most circumstances, it is undesirable to restrict the flow of fluid to or from the jack, as such restriction causes the undesirable generation of heat and the build-up of back pressure in the system which consumes energy more beneficially used to perform other work functions desired of the vehicle. However, it is well known that a negative load condition can cause inlet fluid pressure to the jack to be reduced to a negative amount due to the load causing the jack to move faster than the system can supply fluid to it. This may cause cavitation on the inlet side of the jack and on the various other components of the hydraulic circuit which is equally undesirable.
Many prior art lock valves attempt to minimize such cavitation by modulation of their respective check valves used to prevent the aforementioned leakage so as to restrict fluid flow from the outlet side of the jack to prevent the jack from overrunning the fluid supply to its inlet side. However, due to the difficulties of modulating such check valves and the complex instability problems encountered, the prior art lock valve have not been completely successful in restricting outlet fluid to the minimum flow desired during negative load conditions without, in turn, having a certain amount of unnecessary restriction of fluid flow when maximum flow is desired during positive load conditions.
Thus the prior art lock valves only afford at best a poor compromise between the undesirable effects of cavitation and fluid flow restriction since any benefit gained on the one problem is attained at the expense of the other.